Norwegian coastal waters are darkening due to changes in weather and climate, damaging optical conditions for marine animals.

Marine biologist Dag L Aksnes of the University of Bergen has analysed the impacts of these declining optical conditions with some funding from the Research Council of Norway's research programme on the Oceans and Coastal Areas (HAVKYST). He explained that this occurs when coloured matter from rivers and lakes flows into the sea and mixes with salt water.

Aksnes and his colleagues have been studying the Lurefjorden and Masfjorden fjords in northern Hordaland county on Norway's western coast for many years.

While the fjords are similar and close to each other, Masfjorden contains far more seawater than Lurefjorden, which contains lower-salinity coastal water all the way down to its seabed. Although Masfjorden still has an ecosystem dominated by fish, the darker Lurefjorden has a greater abundance of the jellyfish Periphylla periphylla.

"Periphylla periphylla is a very light-sensitive jellyfish that thrives best in the world's very deep marine waters," continued Aksnes. "But the water in Lurefjorden has now become so murky and dark that it is probably helping this jellyfish to thrive. At the same time, the fjord has become less hospitable as a habitat for important fish species."

Visibility conditions at Lurefjorden have made it more difficult for fish to find their prey, but not for the blind jellyfish. The jellyfish now have virtually no competition for the abundant prey organisms, Aksnes elaborated.

He highlighted that there is a clear correlation between poorer conditions for fish, the increase in jellyfish and the lasting changes in light conditions in the country’s coastal waters.

These conditions also affect algal photosynthesis and the production of organic compounds – which is why the researchers believe that light conditions impact most organisms.

The project demonstrates that changes normally linked to eutrophication (nutrient pollution) and human emissions of nutrients can also cause the water to become darker, reducing the abundance of attached algae such as seaweed and kelp while fueling the growth of planktonic algae.

"More precipitation means that more murky fresh water mixes with the coastal water, making it less saline and murkier," explained Aksnes.

"Furthermore, studies done at the University of Oslo indicate that increased precipitation and rising temperatures lead to changes in vegetation on land, which in turn increases the concentration of CDOM in the fresh water that mixes with the coastal water. We don't know yet whether this leads to undesired changes in our coastal ecosystems, but if so, it will be hard to reverse," the professor added.

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